The term latent images refers to images that are not detectable to the naked eye and are visualized by development. Examples of the latent images include electrostatic images on a photoreceptor before toner is applied to the photoreceptor, latent images on a photographic film, and pictures drawn in invisible ink. The latent images are formed in several ways by using differences in the properties of the surface. Electrostatic images on a electronographic photoreceptor are formed due to differences in electrification. Latent images on a photographic film are formed by formation of a development center in a silver salt crystal. It is thought that similar latent images can be formed due to differences in surface tension.
However, the differences in surface tension were not used to form latent images because a film invisible to the naked eye which desirably generates difference in surface tension was not available. Further, a development procedure to permit detection of the difference in surface tension by the naked eye was not available.
Conventional latent images were deteriorated over time, or were not preserved well. Further, conventional latent images were not returned to the latent form once they were developed.